#include<iostream>
#include<vector>
using namespace std;
typedef vector<int>  ve;  
const int N = 10000;
int main()
{
    int n; cin >> n;
    ve arr;   
    int cnt = 1;
    while (arr.size() < N)
    {
        for (int i = 0; i < cnt; i++)
        {
            arr.push_back(cnt);
        }
        ++cnt;
    }
    int ret = 0;
    for (int i = 0; i < n; i++)ret += arr[i];
    cout << ret << endl;
    return 0;
}